WordNet (r) 3.0 (2006):

sit down
    v 1: take a seat [syn: sit down, sit] [ant: arise, get
         up, rise, stand up, uprise]
    2: show to a seat; assign a seat for; "The host seated me next
       to Mrs. Smith" [syn: seat, sit, sit down]
    3: be seated [syn: sit, sit down] [ant: lie, stand,
       stand up]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "sit down":
   anchor, be seated, billet at, bivouac, boycott, burrow, camp,
   colonize, come to anchor, domesticate, drop anchor, ensconce,
   establish residence, go on strike, go out, hive, hold out, inhabit,
   keep house, light, live at, locate, lock out, moor, move, nest,
   park, people, perch, picket, populate, relocate, remain seated,
   reside, revolt, roost, set, set down, set up housekeeping,
   set up shop, settle, settle down, shut it down, sit, slow down,
   squat, stand, stay at, strike, strike root, take residence at,
   take root, take up residence, touch down, walk out